A portion is what you actually serve yourself. A serving is what the math says you ought to serve yourself, based on nutrition guidelines for healthy women and men. At Cooking Light, we use those guidelines (2,000 calories per day for women and 2,700 for men under 50) to set serving sizes for snacks and meals. Similar math drives the serving sizes you see on food packages.

PLATE X: WHAT SHE SHOULD HAVE EATEN Healthy dinner for Subject A, a woman of average size: 1 cup cooked pasta with meat sauce, 1 ounce garlic bread, and as much salad as would fill out the plate.

Rough as the math is (desk potatoes and marathoners have vastly different requirements), it’s obvious that many Americans eat portions that are bigger than the recommended serving sizes. This isn’t because they can’t guess what a serving looks like: When we told 25 volunteers the amounts that approximate a healthy dinner of pasta, garlic bread, and salad, the majority were able to dish out the correct portion onto a plate (Plate X). However, 70% admitted they would serve themselves more (Plate Y).

PLATE Y: WHAT SHE WOULD HAVE EATEN Subject A told us she knew what the recommended serving sizes were but would normally serve herself what you see here. Y — X = 485 surplus calories.